Time division duplexed multicarrier transmission

ABSTRACT

A digital transmission system comprises a plurality of wire, sets ( 151, 155 ), such as twisted-pair wires for delivering services to subscribers ( 110, 120, 130, 140 ). A first wire set ( 151 ) carries a time-division duplexed multitone signal VDSL. The system is arranged to individually select the transmitted spectra of the upstream and downstream portions of the duplexed signal so as to minimise cross-talk interference with further signals ADSL, HDSL carried by other wire sets ( 155 ). In use, the transmitted spectra of the upstream and downstream portions of the duplexed signal may differ in bandwidth.

TECHNICAL FIELD

This invention relates to telecommunications systems which use digitalmodulation techniques to transport signals.

BACKGROUND OF THE INVENTION

There is an increasing demand for supplying broadband telecommunicationsservices such as high bit-rate data and video-on-demand services tosubscribers. Traditionally the subscriber loop between a subscriber andtheir local telephone exchange has been a twisted-pair cable, withhigher capacity trunks transporting signals between exchanges andconnecting exchanges to service providers. When it is required todeliver broadband services it can be seen that the subscriber loop isthe weak link in the delivery chain. Replacing twisted-pair cables byhigher-capacity optical fibre or coaxial cables provides the requiredimprovement in the subscriber loop capacity but at a prohibitive cost.However, it has been found that by using digital modulation techniquessuch as Discrete Multitone Modulation (DMT) together with digital signalprocessing techniques at the transmitter and receiver, high bit-ratescan be successfully carried over existing twisted-pair subscriber loops.

Several digital transmission schemes are already in use. AsymmetricDigital Subscriber Line (ADSL) provides a high-capacity channel in thedownstream (service provider to subscriber) direction and a lowercapacity upstream control channel in addition to a conventional analoguevoice channel. High bit-rate Digital Subscriber Line (HDSL) provides asymmetrical high-capacity link in both the upstream and downstreamdirections over one or two wire pairs. Both of these schemes usediscrete multitone modulation.

Further research has shown that even higher bit-rates can be carriedover the subscriber loop. Increased bit-rates allows higher-qualityvideo services or an increased selection of services to be provided.Very high-speed Digital Subscriber Loop (VDSL) is a proposed scheme,also using DMT techniques, with an increased capacity of around 25Mbit/s. VDSL occupies the spectral band up to 10 MHz on a subscriberloop.

An article entitled “Performance Evaluation of a MultichannelTransceiver System for ADSL and VHDSL Services” by Peter Chow, Jerry Tuand John Cioffi, published in IEEE Journal on Selected Areas inCommunications, Vol 9, No 6, August 1991, studies the performance ofADSL and VHDSL services including the effects of near-end crosstalk(NEXT) and far-end crosstalk (FEXT).

International Patent Application WO95/28773 (Amati) describes an ADSLcompatible discrete multi-tone transmission scheme that can be used inthe presence of crosstalk noise.

It has been proposed that VDSL use time-division multiplexing (TDM)techniques with duplexing achieved by transmitting in the upstream anddownstream directions in separate time slots. VDSL generally operatesover shorter distances than ADSL. VDSL signals may be coupled on to thesubscriber loop at a street cabinet part-way between an exchange and asubscriber.

TDM schemes which use duplexing are not a good neighbour for othernon-TDM schemes. This poses a problem of compatibility with existing andpossible future schemes when duplexed TDM signals are carried overcables alongside wires carrying signals which are modulated according toother schemes.

SUMMARY OF THE INVENTION

The invention seeks to provide an improved digital transmission system.

According to one aspect of the present invention there is provided amethod of operating a digital transmission system, the method comprisingcarrying on a first wire set a time-division duplexed multitone signal,and individually selecting the transmitted spectra of the upstream anddownstream portions of the duplexed signal whereby to minimisecross-talk interference with further signals carried by other wire sets.

Preferably, in use, the transmitted spectra of the upstream anddownstream portions of the duplexed signal differ in bandwidth.

The spectrum of the upstream portion of the duplexed signal may beselected so as to avoid the frequency bands used by downstream portionsof the further signals carried by the other wire sets whereby tominimise near end cross-talk interference (NEXT) and the spectrum of thedownstream portion of the duplexed signal may be selected so as tooccupy substantially the full bandwidth of the wire set.

The upstream portion of the duplexed signal may occupy the bandwidthabove substantially 1.1 MHz of the wire set to minimise NEXT withdownstream ADSL signals carried by the other wire sets.

Preferably a multitone transmitter/receiver pair couples to each end ofthe first wire set, the step of individually selecting the transmittedspectra of the upstream and downstream portions of the duplexed signalcomprising selectively using multitone sub-channels in the transmitterand the receiver of each pair.

Another aspect of the invention provides a digital transmission systemcomprising a plurality of wire sets, a first of the wire sets carrying atime-division duplexed multitone signal, the system being arranged toindividually select the transmitted spectra of the upstream anddownstream portions of the duplexed signal whereby to minimisecross-talk interference with further signals carried by other wire sets.

Preferably a multitone transmitter/receiver pair couples to each end ofthe first wire set, each pair having control means which individuallycontrols selective use of multitone sub-channels in the transmitter andreceiver.

Preferably each transmitter/receiver pair has a first filter at theoutput of the transmitter and a second filter at the input of thereceiver. This provides additional shaping of the downstream andupstream spectra and minimises the effects of leakage into or fromadjacent multitone sub-channels.

A further aspect of the invention provides a multitonetransmitter/receiver pair for use at one end of a digital transmissionsystem, which pair is arranged to transmit and receive a time-divisionduplexed multitone signal over a wire set, the pair having control meanswhich individually controls selective use of multitone sub-channels inthe transmitter and receiver whereby to individually select thetransmitted spectra of the upstream and downstream portions of theduplexed signal to minimise cross-talk interference with further signalscarried by other wire sets in the system.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to enable a greater understanding of the invention to beattained, and to show by way of example how it may be carried intoeffect reference will be made to the figures as shown in theaccompanying drawings sheets, wherein:

FIG. 1 shows a cabled transmission system;

FIG. 2 shows the allocation of bandwidth to different transmissionschemes which are used in the system of FIG. 1;

FIG. 3 is a block diagram of equipment which performs duplextransmission;

FIG. 4 is a more detailed diagram of a DMT transmitter used in theequipment of FIG. 3;

FIG. 5 shows the output spectrum of a DMT transmitter.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a cabled transmission system linking an exchange 100 to anumber of subscribers 110, 120, 130, 140. Wire sets 151,155, eachdedicated to a particular one of the subscribers, extend from exchange100 to the subscribers. Typically the wire sets are unshieldedtwisted-pair wires.

One of the primary limitations in cabled transmission systems iscross-talk. Signals passing along a first cable will leak to a certainextent into other cables lying alongside that first cable. The two mainforms of cross-talk are near-end cross-talk (NEXT) and far-endcross-talk (FEXT).

NEXT occurs where a high-level transmitted signal is injected into afirst cable, leaks into an adjacent cable and is picked-up at theadjacent cable's receiver located at the end of the cable which is localto the source transmitter. At low frequencies NEXT is tolerable, but athigher frequencies it becomes the dominant source of interference,severely restricting the performance of transmission systems. NEXToccurs at both ends of a cabled system, as shown in FIG. 1.

FEXT occurs where a transmitted signal, injected into a first cable,leaks into an adjacent cable and is picked-up at the adjacent cable'sreceiver located remote from the source transmitter. The level of FEXTincreases with the length of a cable run and can therefore be seen tolimit the reach of a system.

For a major part of the run the wire sets will form part of a cablebundle 150. It is the close physical spacing of wire sets 151, 155 incable 150 and along this first part of the path between exchange 100 andthe subscribers which causes the most significant cross-talk problems.While precautions can be taken to minimise cross-talk, such as byrandomly rearranging the order of wire sets along the length of cable150, cross-talk cannot be eliminated. A path between exchange 100 and ssubscriber premises may be formed by several bundles 150 which arejoined by cross-connection points.

FIG. 1 shows a simplified system where one subscriber transmits andreceives signals which are modulated according to each of the differentschemes. Thus subscriber 110 has signals which are modulated accordingto a VDSL scheme, subscriber 120 has signals which are modulatedaccording to an ADSL scheme, subscriber 130 has signals which aremodulated according to an HDSL scheme and subscriber 140 has a standardPOTS (Plain Old Telephone Service) service. A typical scenario would seevarying numbers of subscribers using each of these different schemes.

Exchange 100 and each of the subscribers have transmitting and receivingequipment to support the schemes. As an example, transmitting/receivingequipment 105 is shown at exchange 100, and complementarytransmitting/receiving equipment 115 is shown at the premises ofsubscriber 110. A high-capacity link, such as an optical fibre cable165, couples transmitting/receiving equipment 105 at exchange 100 to aservice provider 160. Transmitting/receiving equipment 105 s mayalternatively be located at a street cabinet at a cross-connect pointwhich is part-way between exchange 100 and subscriber premises.

FIG. 2 shows how each of the different schemes shown in FIG. 1 make useof the available bandwidth on the wire sets . For clarity, the schemesare show n o n two graphs with a non-linear frequency scale. Each of theschemes will usually be carried over a separate wire set, as shown inFIG. 1, but here are shown together so as to consider the likely effectsof cross-talk between the schemes.

POTS occupies a narrow bandwidth from several hundred Hz to around 4 kHzto carry a duplex analogue telephone signal. HDSL is a symmetricalscheme, with upstream and downstream traffic typically occupying thesame bandwidth—usually up to around 200 kHz. ADSL is an asymmetricalscheme with an upstream signal occupying a band up to around 200 kHz anda downstream signal occupying a much broader band from around 200 kHz to1.1 MHz. Frequency separation of the upstream and downstream signalsminimises NEXT.

The bandwidth of the upstream and downstream signals which form theduplex VDSL scheme are individually chosen so as to minimiseinterference with signals which are modulated according to ADSL and HDSLschemes. Preferably the upstream VDSL signal occupies the bandwidthabove around 1.1 MHz. While this sacrifices some upstream capacity,using separate frequency bands for the upstream VDSL and downstreamADSUHDSL ensures that there is no NEXT between the schemes.

The downstream VDSL signal occupies almost the full bandwidth of thewire set up to 10 MHz in order to maximise downstream capacity. Overlapbetween downstream VDSL and upstream ADSUHDSL (marked A in the figure)is limited to the lower frequency band where NEXT can be tolerated.Downstream VDSL and downstream ADSL share the same frequency band(marked B) but the cross-talk effects are limited to FEXT at the lowerend of the frequency band. The other potential conflict is betweenupstream and downstream VDSL (marked C), but time-division separation ofthe signals in the upstream and downstream directions prevents this.Furthermore, all VDSL sources are synchronised such that they are alltransmitting at the same time or receiving at the same time so as tominimise NEXT between different VDSL wire sets. It can thus be seen thatcross-talk interference is minimised between the schemes.

FIG. 3 shows transmitter/receiver equipment for performing duplextransmission of a multitone signal. Boxes 105 and 115 show thetransmitter/receiver equipment of FIG. 1 in more detail, linked by awire set 151. Equipment 105 and 115 each includes a transmit chain TXand a receive chain RX which are connected to transmission channel 151by a switch SW as required. Switching between transmit and receivefunctions at equipments 105 and 115 at opposite ends of wire set 151 issynchronised as is well-known in TDM duplex systems. Details 300 and 310show an exemplary form of time duplexed transmission at two points alongthe channel, where eight downstream symbols and one upstream symbol forma frame. Other numbers of symbols in each direction could be used. Acommon anti-alias filter 320 can be shared by the transmit and receivepaths, as shown in FIG. 3, or separate anti-alias filters can beinserted into the transmit and receive chains.

Equipments 105, 115 each include a control block 330, 340. Each controlblock outputs a first control signal CTRLM to the modulator MOD and asecond control signal CTRLD to the demodulator DEMOD. The control signalCTRLM fed to the modulator determines (i) the multitone sub-channelswhich are to be used, and (ii) the number of bits which are allocated toeach of the multitone sub-channels. Similarly, the control signal CTRLDinforms the demodulator of the multitone sub-channels which are in useand the number of bits which are allocated to each of those multitonesub-channels.

For transmission in the downstream direction, modulator (MOD) inequipment 105 at a first end of wire set 151 and demodulator (DEMOD) inequipment 115 at the other end of the wire set are controlled so as touse corresponding multitone sub-channels. The selection of multitonesub-channels for use in the upstream direction is independent of thatused in the downstream direction. Modulator (MOD) in equipment 115 anddemodulator (DEMOD) in equipment 105 are also controlled so as to usecorresponding multitone sub-channels.

The particular selection of sub-channels which is used is dependent ontwo constraints. Certain sub-channels may be forbidden a-priori, such asthose used by downstream ADSL. The transmitter which transmits, and thereceiver which receives the upstream portion of the duplex signal areboth forbidden from using these channels as they are known to causeexcessive cross-talk interference. The control blocks 330, 340 inequipments 105, 115 may store these forbidden channels in a permanent orprogrammable memory.

Other sub-channels may be not be used when it is discovered at start-upthat they have insufficient signal-to-noise ratio (SNR) to supportreliable transmission of data. During the start-up process the SNRpredicted for a particular sub-channel is used to allocate a number ofbits to be carried on that sub-channel. This is not known a-priori, butis based on frequency dependant SNR measurements, and may also beinformed by knowledge of the worst case interference regime. During thestart-up process, the allocation of the number of bits per sub-channelwhich the receiver is to use is made known by the transmitter.

FIG. 4 shows a typical transmitter chain in more detail. It comprises aninput bit stream buffer and encoder, an inverse fast Fourier transformunit, a serial formatting unit (P/S), a digital-to-analogue converter(DAC) and an output low pass filter (LPF). In such an arrangement, aninput bit stream of data rate R bps is buffered into blocks of T_(b)bits by the buffer, where T_(b) is the total number of input bits permodulator symbol. T_(b) is given by the product of the data rate and thesymbol period (T) of the DMT modulator. These T_(b) bits are dividedamongst the sub-channels, each having b_(i) bits.

These b_(i) bits for each of the N_(s) sub-channels are translated inthe DMT encoder into a complex sub-symbol S_(i), for each sub-channel.Each subchannel has 2^(bi) possible QAM symbol states. The 2 N_(s) pointIFFT unit combines the Ns subsymbols into a set of N real-valued timedomain samples, X_(n, t); where n=1 . . . N, and t represents the sampletime. These N samples are successively applied (in serial format) to adigital-to-analogue converter, DAC, which samples at a rate N/T—which isthe sampling rate of the DMT modulator—to create a symbol for the DMTmodulator. The output of the DAC is a continuous-time modulated signalx(t) which is a succession of DMT symbols each formed from N time domainsamples.

The following key relates to FIG. 4 of the drawings:

(a) Ns QAM symbols-drive only half the IFFT inputs e.g. 256 symbols (b)N time domain samples-uses only real output of samples e.g. 512 samples.P/S Parallel to serial converter time index for each IFFT cycle N = 2NsT Symbol period of DMT modulator NFF Sampling rate of DMT modulator

An idealised output spectrum of the DMT transmitter of FIG. 4 is shownin FIG. 5. The sub-channels are centred on F_(i)=i/T.

In practice, some energy leaks from sub-channels according to therelationship sin(f−F_(C))/(f−F_(C)), where F_(C) is the DMT sub-channelfrequency in question. Use of an improvement detailed in InternationalPatent Application No. PCT/GB96/02008 can reduce this leakage.

A further way of reducing the effects of the leakage of energy fromsub-channels is to use additional filters in the transmitter andreceiver. Referring again to FIG. 3, filter 351 is coupled to the DACport of the transmitter, and filter 352 is coupled to the ADC port ofthe receiver. Similar filters 353, 354 are shown in equipment 115 at theother end of wire set 151. These filters additionally shape thebandwidth of the transmitter and receiver. The transmitter filters 351,354 minimise leakage of the transmitted signal into adjacent bands. Thereceiver filters 352, 353 reduce the effects of leakage of signals fromadjacent bands into the wanted band.

Preferably the power spectral densities of the duplex VDSL system andADSL schemes are made equivalent.

What is claim is:
 1. A method of operating a digital transmission systemcomprising a plurality of wire sets, the method comprising carrying on afirst of the wire sets a time-division duplexed multitone signal, andindividually selecting the transmitted spectra of the upstream anddownstream portions of the duplexed signal whereby to minimisecross-talk interference with further signals carried by other wire setsof the plurality of wiresets by ensuring frequency separation ofupstream TDD signals from downstream signals of frequency separation ofupstream TDD signals from downstream signals of the further signalswhile permitting frequency overlap of downstream TDD signals with thefurther signals.
 2. A method of operating a digital transmission systemaccording to claim 1 wherein the transmitted spectra of the upstream anddownstream portions of the duplexed signal differ in bandwidth.
 3. Amethod according to claim 1 wherein the transmitted spectra of theupstream and downstream portions of the duplexed signal are overlapping.4. A method of operating a digital transmission system according toclaim 1 wherein the spectrum of the upstream portion of the duplexedsignal is selected so as to avoid the frequency bands used by downstreamportions of the further signals carried by the other wire sets wherebyto minimise near end cross-talk interference (NEXT) and the spectrum ofthe downstream portion of the duplexed signal is selected so as tooccupy substantially the full permitted bandwidth of the wire set.
 5. Amethod of operating a digital transmission system according to claim 4wherein the upstream portion of the duplexed signal occupies thebandwidth above substantially 1.1 MHz of the wire set to minimise NEXTwith downstream ADSL signals carried by the other wire sets.
 6. A methodof operating a digital transmission system according to claim 1 whereina multitone transmitter/receiver pair couples to each end of the firstwire set, the step of individually selecting the transmitted spectra ofthe upstream and downstream portions of the duplexed signal comprisingselectively using multitone sub-channels in the transmitter and thereceiver of each pair.
 7. A method according to claim 6 wherein the stepof individually selecting the transmitted spectra comprises referring toa store of forbidden channels stored in a memory at thetransmitter/receiver.
 8. A method according to claim 6 wherein the stepof individually selecting the transmitted spectra is based on frequencydependent signal-to-noise ratio (SNR) measurements.
 9. A digitaltransmission system comprising a plurality of wire sets, a first of thewire sets carrying a time-division duplexed multitone signal, the systembeing arranged to individually select the transmitted spectra of theupstream and downstream portions of the duplexed signal whereby tominimise cross-talk interference with further signals carried by otherwire sets of the plurality of wiresets by ensuring frequency separationof upstream TDD signals from downstream signals of the further signalswhile permitting frequency overlap of downstream TDD signals with thefurther signals.
 10. A digital transmission system according to claim 9wherein a multitone transmitter/receiver pair couples to each end of thefirst wire set, each pair having control means which individuallycontrols selective use of multitone sub-channels in the transmitter andreceiver.
 11. A digital transmission system according to claim 10wherein each transmitter/receiver pair has a first filter at the outputof the transmitter and a second filter at the input of the receiver. 12.A digital transmission system according to claim 9 wherein the wire setscomprise twisted-pair wires.
 13. A multitone transmitter/receiver pairfor use at one end of a digital transmission system comprising aplurality of wire sets, which pair is arranged to transmit and receive atime-division duplexed multitone signal over a first of the wire sets,the pair having control means which individually controls selective useof multitone sub-channels in the transmitter and receiver whereby toindividually select the transmitted spectra of the upstream anddownstream portions of the duplexed signal to minimise cross-talkinterference with further signals carried by other wire sets of theplurality of wiresets by ensuring frequency separation of upstream TDDsignals from downstream signals of the further signals while permittingfrequency overlap of downstream TDD signals with the further signals.14. A telecommunications network for delivery of broadband services tosubscribers over twisted-pair subscriber loops incorporating a systemaccording to claim 9.